Computer aided detection of tissue structures of interest is becoming increasingly important in radiology, in particular in mammography. Computer aided detection corresponds to a technique that detects suspicious regions within an acquired X-ray image by comparing visible feature shapes to a reference library. In general, spatial information, i.e. feature shape, is evaluated without respect to absolute values in the image, i.e. a signal strength. In particular in mammograms, the occurrence of anatomical noise is limiting the ability to detect lesions. Contrast from both anatomical noise and suspicious lesions decrease with photon energy and, depending on the lesion type, its visibility may be optimized at a certain photon energy. E.g., tumor detectability is improved at higher energies while micro-calcifications may preferably be detected at lower energies. In particular, tumors are heavily affected by anatomical noise and the decrease in anatomical noise at higher energies outweighs the decrease in tumor contrast, whereas microcalcifications are less affected by anatomical noise and reducing the amount of anatomical noise has little impact.
Accordingly, employing image information, which is acquired at a specific photon energy is referred to as spectral imaging or employing spectral information. The energy E of a single photon is inversely related to its wavelength λ by the formula
  E  =            h      *      c        λ  (with E: energy, h: Planck's constant, c: speed of light).
Spectral imaging employs the detected energy spectrum to extract information about the material contents of the object. The technique has the potential to improve detection also at screening without contrast agent, i.e. unenhanced imaging, which however may increase the amount of quantum noise present in image information.
B. Norell, E. Fredenberg, K. Leifland, M. Lundqvist, B. Cederström, “Lesion characterization using spectral mammography” SPIE Medical Imaging 2012: Physics of Medical Imaging describes using spectral image information for characterizing and distinguishing detected structures in an X-ray image.